Here on Arch Linux, usr/share/man/man5/fonts-conf.5.gz is owned by extra/fontconfig 2.13.0+10+g58f5285-1. It has several cases of /<test.*mode=/, e.g. `<test name="family" mode="eq">'. My config files have errors with that and I have to s/mode/compare/. There's a couple of `compare=' in the man page.
https://www.freedesktop.org/software/fontconfig/fontconfig-user.html has the same problem; a mixture of `mode' and `compare' for specifying <test>'s operator.
Thanks. fixed in git c78afa9